Modern techniques for maximizing the recovery of energy from fossil fuels, or for clean use of low grade fossil fuels, call for fluidized bed gasification and combustion installation. The fluidized bed fuel process typically involves a flow of gases through a bed of solid particles at a velocity sufficiently high to support the weight of the particles but not high enough to carry them out of the bed. Free movement of the particles and high velocity between gas and solids do promote rapid heat transfer. Moreover, flow and handling of the solids constitute an important factor for attaining good efficiency and maintaining the continuity of the reaction process. As a matter of fact, it is the ease and versatility in solid flow handling, and the large amount of solid surface in contact with flowing gas for a relatively small volume, which make the fluidized bed commercially attractive.
Bed temperatures are also very important for monitoring and control, since the free movement of particles favors heat transfer both within the bed and between the bed and adjoining surfaces.
Thus, it is important in a fluidized bed that solid materials can be readily added to, or removed from, a fluidized bed while the counteracting circulation of solids and gas is being maintained at all times with the required intensity.
An object of the present invention is to detect stagnant regions in a fluidized bed. Another object of the present invention is to provide a probe which can be applied to a pneumatic conveying line for the detection of stagnancy of solid particles.
It is known to measure the size of solid particles and their concentration in a dust-laden gas. For instance, in U.S. Pat. No. 3,679,973 the dust particles are initially ionized, then conveyed with the stream of gas into a chamber for measuring the electrical discharge thereacross. Calibration is provided for different particle sizes and different particle concentrations.
It is known from U.S. Pat. No. 3,939,694 to measure the concentration of solid particles suspended in a gas phase of the aerosol or fume type, by passing the aerosol through a fluidized bed of balls whose diameters are greater than those of the largest dust particles, then measuring the intensity of currents flowing from the walls housing the bed. Such current is characteristic of a charge distribution between balls and walls due to collisions between particles.
It is known from U.S. Pat. No. 3,636,763 to electrically measure the flow rate of particular material in a pneumatic conveyor by fixing an insulated electrode across the wall thereof to detect a capacitance effect influenced by mass flow.
The generation of electrostatic charge in a gas fluidized bed is well known. See Boland, D. and Geldart D., in Powder Technology, Vol, 5, Page 289 (1971-1972). The spontaneous generation of electric charges in a pneumatic conveying line has also been reported. See Richardson, J. F. and McLeman, M. in Transaction Institute Chemical Engineers, Vol. 38, Page 257 (1960).
It is also known that dust ignitions might occur spontaneously in a fluidized bed, because of spark discharges. See Boyle, A. R. and Llewellyn J. in Journal Society Chemical Industry, Vol. 69, Page 173 (1947). Indeed, charge generation during fluidization has been recognized as a potential safety hazard.
The attraction between charged particles is known to interfere with the normal hydrodynamics of a fluidized bed. It may affect particle motion and momentum transfer in dilute, turbulent gas solids flow systems. As a result of mutual collisions among particles and between particles and walls positively and negatively charged, surfaces are built up in a fluidized bed through electron transfer. The probability of electron transfer following collision depends primarily upon the properties of the materials, the physical state of their surfaces, and the energy of their impact. A body delivering electrons more readily will become positively charged and vice versa. Thus the prime requirement for static electrification is the making and breaking of surface contact. It follows that electrostatic generation can occur between collisions of conductor and insulator, insulator and insulator, and conductor and conductor although the degree of electrification may be different.
Since a charged particle is essentially a capacitor, it can be charged, or discharged, in contact with other particles, or walls, when the potential difference between the two surfaces is large enough to overcome the contact resistance. Based on these notions, efforts have been made to regulate the electrostatic charge. This can be done only through: (1) changing the effective time of surface contact, and (2) changing the properties of the surface layer.
The consensus in the prior art has generally been towards lowering the electrostatic charge in a fluidized bed. To this goal, the most common methods suggested are: increasing relative humidity in the fluidizing mediums, treating particle surface with conducting films, air ionization and grounding of conducting parts. In fact, these attempts in industry have not been successful. Actually, an electrostatic charge is always present wherever gas-solids flow systems are involved. The present invention rests on a positive approach rather than a negative approach to electrical charging in a fluidized bed.